A number of attempts have been made to manufacture printed circuit boards using powdered metal. An example is found in U.S. Pat. No. 4,614,837, which discloses a three-step process. First, metal particles of less than 75 microns in size are applied over the surface of a substrate made of a dielectric material which softens on application of heat, hardens on removal of heat and is not materially affected by the softening and hardening process. Second, a die formed into the desired circuit pattern is heated and pressed against the powdered surface of the substrate at high pressure (20,000-50,000 psi) and room temperature to compact the powder underneath the die head into the desired circuit pattern on the substrate. Third, the die is heated to 200.degree. C. and applied at lower pressure (10,000 psi) to the substrate for 20-60 minutes, thereby embedding the compacted powder into the surface of the substrate to a depth of approximately 75 microns.
Another process for making printing circuit boards from metal powders is disclosed in U.S. Pat. No. 4,698,907. The first step in this process is to form a master pattern of the desired circuit. Next, conductive metal particles having a thin, non-conductive coating are applied to the master pattern by electrostatic forces. A non-conductive substrate is then placed close to the master pattern and electrostatic forces transfer the conductive powder from the master pattern onto the substrate in the desired pattern. While this patent discloses the need to fuse the particles in order to complete manufacture of a low resistance electrically conductive path, it does not provide any details as to how this can be accomplished. One method for doing so, however, is shown in U.S. Pat. No. 5,014,420, which discloses a method of fusing the positive particles using an intense, highly-concentrated high-frequency electromagnetic field.
A number of additive processes for making printed circuit boards require that the metal particles be mixed with a viscous carrier to ensure that the particles remain in the desired pattern on the substrate during the curing process. For example, U.S. Pat. No. 2,993,815 discloses application of a paste containing copper particles in glass frit to a refractory substrate in the desired pattern. The substrate is then heated in an oven to sinter the metal, causing it to bond to the substrate. In U.S. Pat. No. 4,278,702, a silk screen is used to apply a paste comprising powdered metal particles and binder to the substrate in the desired circuit pattern and a high frequency magnetic field is then used to sinter the particles at a sufficiently low temperature that the substrate is not warped or melted.
Each of the foregoing prior art processes suffers from a number of disadvantages. For example, in those in which the metal particles are suspended in a viscous carrier, the carrier must be driven off during the curing process in order to obtain a satisfactory conductive trace. This complicates the manufacturing process because the metal particles must first be mixed into a suitable carrier and the carriers must then be driven off prior to curing the metal particles, the latter of which takes substantial time relative to the time needed for curing. These processes also require the use of screen printers, which substantially increase the cost of equipment needed to manufacture printed circuit boards.
The prior art processes for making printed circuit boards directly from metal particles also suffer from a number of disadvantages. First, these processes require the use of expensive machinery for applying the metal particles in the desired circuit pattern on the substrate, e.g., an electrostatic printer or a die capable of exerting high pressures on the substrate. Second, these processes are only suitable for sintering the metal particles or require that the metal particles first be coated with a non-conducting material before being cured.
Accordingly, one object of the present invention is to provide a method and apparatus for depositing metal particles on a dielectric substrate in a predetermined circuit pattern without need of wetting agents or binders.
Another object of the present invention is to provide an apparatus and method for depositing metal particles on a dielectric substrate in a desired circuit pattern without a screen printer, electrostatic printer or other similar printing apparatus.
Yet another object of the present invention is to provide a method and apparatus for depositing metallic particles on a dielectric substrate in a desired circuit pattern so that they may thereafter be melted into continuous conductors in the desired pattern without having to first drive off any carriers or wetting agents.
A further object of the instant invention is to provide a method and apparatus for depositing metal particles on a dielectric substrate quickly and inexpensively.
Still another object of the instant invention is to provide a method and apparatus for depositing metal particles on a dielectric substrate so that such particles can be rapidly cured.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.